Evolution and systematics

The Cheloniidae are most closely related to the leatherback turtles of the family Dermochelyidae. Fossils are known from as early as the Cretaceous period. Because of uncertainty about the phylogenetic relationships among the five living genera, no subfamilies are currently recognized.

Physical characteristics

Large marine turtles with a low streamlined shell covered with scutes, paddle- or flipper-like forelimbs, a skull with a

solid bony roof, and a head that cannot be retracted within the shell. The plastron is somewhat reduced, lacks a hinge, and is connected to the carapace by ligaments.

Distribution

All tropical and subtropical oceans. Some seaturtles range well into temperate seas, including the Mediterranean Sea.

Habitat

Coastlines on the continental shelves, where feeding and nesting sites are most abundant. Hatchling and small juveniles of most species apparently are mostly pelagic.

Behavior

Seaturtles (males and females) often make extremely long migrations between feeding and nesting grounds (at least 190 mi [300 km] in some cases). Some species congregate off the sandy nesting grounds and then nest en masse in large groups called "arribadas." Most nesting is done at night, but one species is a diurnal nester. Seaturtles enter temperate seas during the summer but usually either migrate to warmer waters or bury themselves in the mud in shallow coastal areas for the winter.

Feeding ecology and diet

All but one species of seaturtles are primarily carnivorous, feeding on sponges, mollusks, crustaceans, barnacles, sea urchins, or fish. The green seaturtle is primarily herbivorous, feeding mainly on sea grasses.

Reproductive biology

beaches, producing as many as seven or more clutches in a season at intervals of 10–30 days. The eggs are leathery and spherical and measure 1–2 in (30–60 mm) in diameter. Clutches usually contain 90–130 eggs, although maximum clutch size can approach 250. Incubation is generally quite short, only 40–70 days. All species exhibit temperature-dependent sex determination. Warm temperatures produce mostly females, and cool temperatures produce mostly males.

Conservation status

Five species of seaturtles are classified as Threatened, with two listed as Critically Endangered, three as Endangered. The remaining species is listed as Data Deficient.

Significance to humans

Despite international legislation to protect them, seaturtles and their eggs are still eaten locally around the world. The shells of some species, particularly the hawksbill (Eretmochelys imbricata), are used for making trinkets. Many more turtles are accidentally killed in fish and shrimp nets. Turtle excluder devices placed on these nets are known to reduce seaturtle drowning by at least an order of magnitude and are legally required by many countries, but universal use is sorely needed. Increasing numbers of seaturtles are being found with fibrous tumors (fibropapilloma) up to 12 in (30 cm) in diameter on the skin, in the mouth, and on the internal organs. More than 70% of the turtles in some areas are infected. The cause of this mysterious disease is not yet fully understand, although it is contagious and certainly linked to human pollution. The long-term effects of these tumors on seaturtle populations are unknown.

Species accounts

List of Species

taxonomy

Testudo caretta Linnaeus, 1758, Insulas Americanas ("American islands"). No subspecies are currently recognized, although the Pacific and Atlantic populations have been considered different races by some authors.

other common names

physical characteristics

The loggerhead seaturtle is the largest species in this family, reaching 84 in (213 cm) carapace length and weights up to 1,000 lb (454 kg). The head is quite broad posteriorly and short and round in front, hence the common name. Two pairs of prefrontal scales are present on the top of the head forward of the eyes. The heart-shaped carapace is serrate posteriorly and has five or more pairs of pleural scutes, the first pair in contact with the nuchal scute. Eleven to 15 (typically 12 or 13) marginal scutes are present on the rim of the shell. Three inframarginal scutes (all lacking pores) are present on the bridge between the marginal and the plastral scutes.

distribution

All tropical and temperate seas but rare in the eastern and central Pacific Ocean.

habitat

Mainly shallow marine waters along tropical continental shores but also around some islands. Loggerhead turtles enter bays, creeks, salt marshes, and the mouths of rivers.

behavior

Loggerhead turtles undertake long migrations, often using warm oceanic currents for dispersal. A juvenile released off Okinawa Island was recaptured off San Diego just over two years later, and several adults have been recaptured 1,300–1,700 mi (2,100–2,700 km) from the site of original capture. Females migrate to nesting areas every two or three years. Adults often aggregate off nesting beaches before migrating

back to feeding habitats. Hatchlings and small juveniles are apparently pelagic and associated with floating plants, animals, and flotsam. This species often ventures into temperate waters and nests farther north than any other seaturtle (solitary nests have been found in New Jersey in the United States). When in open waters, loggerhead seaturtles often float on the surface, presumably sleeping. Mitochondrial DNA studies have shown that turtles from different nesting regions differ genetically. This finding suggests that females return to the nesting beaches on which they hatched.

feeding ecology and diet

The loggerhead turtle is primarily carnivorous throughout its life. Hatchlings are known to eat jellyfish, snails, crustaceans, insects, and sargassum (an alga), most obtained while the turtle is floating in sargassum mats. Juveniles and adults feed mostly on the bottom and eat sponges, worms, conch and other snails, clams, squid, octopus, barnacles, horseshoe and other crabs, shrimp, sea urchins, fish, and occasionally hatchling seaturtles, algae, and other aquatic plants.

reproductive biology

Loggerhead turtles reach maturity between 10 and 30 years of age. Courtship and mating apparently occur most commonly during the migrations to nesting grounds, several weeks before nesting begins, rather than near the nesting beaches. The male circles the female, bites her neck and shoulders, and mounts her shell from behind. The pair typically floats at the surface during copulation. Mating can occur day or night. Females apparently mate several times. DNA studies have revealed that more than one male may father eggs laid in a single clutch. Nesting usually occurs in spring and summer but with great geographic variation, particularly latitudinal, in timing and duration.

Nests are generally excavated above the high tide line, in front of the first dune, and usually at night. Once the site is chosen, the female first excavates a body pit using all four limbs and then uses only her rear feet to dig the nest chamber in the bottom of the pit. She then deposits 23–198 spherical, leathery eggs (usually 95–130) that measure 1–2 in (2.5–5.1 cm) in diameter and 1–2 oz (26–47 g) in mass. She then covers the nest, first using only her hind legs and eventually using all four limbs to cover and camouflage the entire site. Females may lay up to seven clutches per season at intervals of nine to 28 days, most typically about every two weeks, although four or five clutches per season is more usual. Most females nest only every two or three years. Incubation requires 46–80 days, typically 60–65, depending on the temperature. Hatchlings generally emerge from nests at night to avoid lethal ground temperatures during the day. They then scurry immediately to the surf. This species has temperature-dependent sex determination. Mostly females are produced above 84–86°F (29–30°C), and mostly males are produced below this temperature.

conservation status

Loggerhead turtles are classified as Endangered on the IUCN Red List.

significance to humans

Although direct consumption of adults and eggs may be declining in many areas, humans are still responsible for much indirect mortality among loggerhead seaturtles through activity on or development of nesting beaches, by contributing to increases in predators such as raccoons and dogs, by drowning the turtles in shrimp or fish nets, and with pollutants.

Green seaturtle

Chelonia mydas

taxonomy

Testudo mydas Linnaeus, 1758, Insulas Pelagi: Insulam Adscensionis ("Oceanic Islands: Ascension Island"). No subspecies are currently recognized. Nevertheless, some authors recognize the Pacific green turtle (also called the black turtle) as a distinct species; others consider the Pacific green turtle a subspecies of Chelonia mydas. Results of DNA studies do not support recognition of the Pacific turtle as distinct from other green seaturtles.

other common names

physical characteristics

Large, reaching 60 in (152 cm) carapace length and 750 lb (340 kg) body mass. The head is small and rounded anteriorly, and only one pair of elongated prefrontal scales is present on the top of the head forward of the eyes. The heart-shaped carapace is only weakly serrate posteriorly and has four pairs of pleural scutes, the first pair of which does not contact the nuchal scute. Twelve marginal scutes are typically present along each side of the shell. Four inframarginal scutes (all lacking pores) are present on each bridge between the marginal and the plastral scutes. The greenish color of the fat of this turtle is the source of its common name.

distribution

All tropical and temperate seas.

habitat

Although green seaturtles venture into temperate seas, adults are found primarily in the tropics. These turtles can be found in the open sea, but they are most commonly seen in areas of shallow water with an abundance of submerged vegetation, especially sea grass flats. Hatchlings are more pelagic and often are found in mats of sargassum.

behavior

Green seaturtles nest primarily on tropical beaches and may migrate more than 1,900 mi (3,000 km) between feeding and nesting areas. These turtles are known to thermoregulate by basking at the water's surface, but they are the only marine turtle known to leave the water to bask on land. One population in the Gulf of California is known to hibernate under water by partially burying itself in the substrate.

feeding ecology and diet

Although it is assumed that hatchlings and juveniles are primarily carnivorous, few data are available. Adults are well known to be almost completely herbivorous, feeding primarily on several genera of sea grasses as well as on algae. Animal matter, such as sponges, jellyfish, mollusks, crustaceans, sea urchins, and sea squirts, is occasionally ingested, some perhaps secondarily while the turtle feeds on sea grasses. Feeding generally occurs during the day.

reproductive biology

The age at maturity of green seaturtles is not definitively known but is speculated to be between 20 and 30 years. Courtship and mating take place off the nesting beaches, and females may mate with several males. Courtship involves chasing, nuzzling, rubbing, sniffing, and biting the female. If the female is receptive, the male mounts her shell from behind and swings his much larger tail under hers for intromission. Copulation may last several hours; one report describes a 52-hour mating. Mating may occur at or below the water's surface. Females can store sperm, perhaps for several years, so individual eggs in the same clutch may have different fathers.

There is considerable variation in the timing of the green seaturtle nesting season, both locally and globally. For example, in the western Atlantic, nesting is typically from March to October, with a peak from May to September. In the eastern Pacific, nesting may occur any time between February and January. Females exhibit considerable fidelity to their nesting beaches, and this trait accounts for slight genetic differences between separate nesting colonies. Females nest anywhere from the open sand above the high tide line to fully shaded areas just inland from the beach. Most nesting occurs at night. Nest construction is similar to that of the loggerhead seaturtle, except that the green seaturtle digs the deepest body pit of any cheloniid sea turtle (up to 20 in [50 cm]). Females lay as many as seven clutches in a season, usually at 12–14-day intervals, but laying two to five clutches is most common. Clutch size is generally positively related to the female's size. Clutches range from three to 238 eggs, although clutches of 100–120 eggs are typical. The leathery eggs are nearly spherical and 1–2 in (2.5–5.8 cm), usually 1.4–1.6 in (3.5–4 cm) in diameter and weigh 1–2 oz (28–65 g, usually 35–50 g). Most females nest only every two years, but cycles of one to four years are known. Incubation typically requires 50–70 days but may take between 30 and 90 days, depending on nest temperature. Hatchlings emerge from their nests at night and move immediately to the sea. Green seaturtles exhibit temperature-dependent sex determination. High temperatures produce mostly females, and low temperatures produce mostly males.

conservation status

Green seaturtles are classified as Endangered on the IUCN Red List. The Mediterranean subpopulation is listed as Critically Endangered.

significance to humans

Despite international protection, green seaturtles and their eggs are still consumed by local peoples in many parts of the world. Many other human activities also increase mortality. Turtles are killed for sport, drown in shrimp or fish nets, and are wounded by boat propellers. Other turtles are killed by predators whose numbers have increased because of human activities (e.g., raccoons, pigs, and dogs). Nesting grounds are destroyed by hotel and housing developments.

other common names

physical characteristics

The Kemp's ridley is the smallest of the seaturtles, reaching a maximum of only 30 in (76 cm) carapace length, and 108 lb (49 kg) body mass. The head is somewhat pointed anteriorly and has a distinctly hooked upper beak. Two pairs of prefrontal scales are present on the top of the head forward of the eyes. The heart-shaped carapace is serrate posteriorly and has five pairs of pleural scutes; the first pair is in contact with the nuchal scute. Twelve to 14 marginal scutes are present on the rim of the shell. Four inframarginal scutes (each with a posterior pore) are present on each bridge between the marginal and the plastral scutes.

distribution

habitat

Adult ridleys prefer the shallow water of the Gulf of Mexico, although for the first two years of life they drift in floating mats of sargassum or other flotsam in the gulf currents. Subadults venture into temperate waters, such as Chesapeake Bay, to feed.

behavior

Little is known about the aquatic behavior of Kemp's ridley turtles. Although mostly confined to the Gulf of Mexico, females still may migrate long distances, often more than 600 mi (1,000 km) to and from the only significant nesting beach, at Rancho Nuevo, Tamaulipas, Mexico.

reproductive biology

Age at maturity of Kemp's ridley turtles is estimated to be 8–12 years. Females then reproduce at one- to three-year intervals, most nesting every one or two years. Courtship and mating occur off the nesting beaches before nesting. These behaviors have not been well described but are similar to those of other seaturtles. Nearly all nesting occurs along a single beach in Tamaulipas, Mexico, from mid April to mid July. Females nest almost exclusively during the day, coming ashore in arribadas, although some come ashore alone. Nests are dug in open sand on the upper beach or in the dunes behind the beach. Nest construction is similar to that by other cheloniid sea turtles, but the Kemp's ridley nests are generally shallower. Females produce up to four clutches, usually one to three, per season at intervals of 10–49 days, although the usual interval is 20–28 days. Clutch size ranges from 51 to 185 eggs, nests of 100–110 eggs being most common. The spherical, leathery eggs measure 1–2 in (2.5–5.1 cm, averaging 3.9 cm) in diameter and weigh 1–1.5 oz (24–41 g, averaging 30 g). Incubation requires 45–70 days depending on temperature, but most eggs hatch in 50–55 days. Kemp's ridley turtles exhibit temperature-dependent sex determination. High temperatures produce mostly females, and low temperatures produce mostly males.

conservation status

Kemp's ridley turtles are classified as Critically Endangered on the IUCN Red List.

significance to humans

Kemp's ridleys once were the most abundant seaturtle in the Gulf of Mexico, an estimated 42,000 nesting in Mexico in one day. Although these turtles are rigidly protected internationally and the primary nesting beach is a Mexican national reserve, Kemp's ridley turtles remain the most critically endangered marine turtle, perhaps 1,000–2,000 adults remaining. Drowning in shrimp trawl nets is believed to be the most prevalent unnatural cause of death. Recent increases in the nesting population offer hope for recovery.

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